In wireless communication systems multiple antennas are often used in base stations and terminals to increase the coverage, especially for high data rate services in wide area scenarios. The multiple antennas help achieve this by introducing the possibility of beam forming as well as spatial diversity and efficient provisioning of very high data rates by means of spatial multiplexing. Beam forming, in particular, is used to increase the quality of a particular communication link.
When a terminal wishes to set up a connection to the network, it is important to select a link that will provide adequate communication quality. To assist selection of communication link in a system having multiple antennas according to the prior art cell defining reference signals (pilots) are transmitted. The base station typically transmits a cell defining pilot, typically consisting of one or two reference signals with a fixed set of beam forming weights. For example, each reference signal may be transmitted from one of the antennas. This pilot signal is transmitted continuously or periodically. The mobile terminals perform measurements on this pilot signal. Based on these measurements a terminal can determine which cell it hears best and select this cell, or, when needed, initiate a handover by sending a measurement report to the network.
The pilot signal is chosen so that it can be received in a large area by all visiting terminals. Typically, a cell defining pilot is transmitted in a wide beam covering a 120 degree sector. In contrast, with beamforming, the dedicated data intended for one particular terminal is typically transmitted in narrower beams. There may then a mismatch between the quality of the cell defining pilot and the quality of the communication link using beamforming.
Another example, is Wideband Code Division Multiple Access (WCDMA) with transmit diversity. Here, cell selection is based on measurements of reference signals transmitted from two antennas over the entire cell. The terminal then adds the measurements of the reference signals corresponding to the individual antennas to use as input data for the selection of an appropriate cell. This again means that the quality of the pilot signal used to select the base station with which to communicate may differ significantly from the quality of the communication link once the cell has been selected when beamforming is used. This, since the cell selection is based on reference signals that do not reflect the properties of the communication link that will result if beam forming is used.